1. Field of the Invention
The present invention relates to an organic laminated photosensitive material of the positive charging type which has a high saturation charge voltage, an increased sensitivity and an excellent printing resistance.
2. Description of the Prior Art
A photosensitive material for the electrophotography comprising an electroconductive substrate and a layer of an inorganic or organic photoconductor formed on the substrate has been widely used. As a kind of this photosensitive layer, there is known a so-called function-separated organic photosensitive material in which a charge-generating substance and a charge-transporting substance are combined by lamination or dispersion.
Most of known function-separated organic photosensitive materials are of the negative charging type, but photosensitive materials of the negative charging type involve a problem in that ozone is generated at the time of charging. Accordingly, organic photosensitive materials of the positive charging type are eagerly desired.
As the organic photosensitive material of the positive charging type, there is known an organic photosensitive material comprising a charge-generating layer laminated on a charge-transporting layer having a hole-transporting property. In the organic photosensitive material, if the thickness of the carrier-generating layer (charge-generating layer) is not reduced, injection of charges is not sufficiently performed, and if the thickness of the charge-generating layer is reduced, the abrasion resistance is poor and the printing resistance is degraded.
As means for overcoming this defect, Japanese Patent Application Laid-Open Specification No. 92962/87 discloses a photosensitive material comprising a carrier-generating layer (charge-generating layer) comprising anthanthrone bromide as the carrier-generating substance, a carrier-transporting substance and a binder resin, in which the content of anthanthrone bromide is 10 to 300% by weight based on the binder, the content of the carrier-transporting substance is 20 to 200% by weight based on the binder and the carrier-transporting substance/anthanthrone bromide weight ratio is from 0.2 to 3.0. It is taught and known that in this photosensitive material, a highest sensitivity is obtained if the thickness of the carrier-generating layer is smaller than 5 .mu.m.
This known photosensitive material is significant in that by incorporating the charge-transporting substance in the charge-generating layer, injection of holes in the charge-transporting layer can be performed smoothly even if the topmost charge-generating layer is relatively thick. However, the thickness of the charge-generating layer is still limited from the viewpoint of the sensitivity, and it has been confirmed that if the thickness exceeds the level giving the maximum sensitivity, the sensitivity is drastically reduced.
As pointed out hereinbefore, increase of the thickness of the topmost charge-generating layer is preferred for improving the printing resistance of the photosensitive material and increasing the surface saturation charge voltage (increasing the contrast). Therefore, development of a photosensitive material having an increased sensitivity irrespectively of increase of the thickness of the charge-generating layer is eagerly desired in the art.